1. Field of the Invention
The present invention relates to a prepreg with thermosetting resins as a matrix for producing fiber-reinforced composite materials which can give excellent toughness to the resulting molded product without injuring excellent handling property and thermal and mechanical properties inherent to such the prepreg, a process for producing the same and laminates produced with the same.
2. Description of the Related Art
Advanced composite materials with high-strength, high-elastic modulus fibers (e.g. carbon fiber) as a reinforcing material have been widely used chiefly in the field of sports by effectively using their characteristics of being excellent in specific strength and specific elastic modulus.
These advanced composite materials are supplied and used in the form of an intermediate material generally called a prepreg, so that resins usually used as a matrix resin are thermosetting resins which are easy to give moderate tackiness and flexibility (draping property) necessary to laminate the prepregs.
While, however, thermosetting resins represented by epoxy resins have various characteristics of being excellent in heat resistance, solvent resistance, mechanical properties, etc. in addition to the foregoing characteristics, they have defects of being inferior in impact resistance because of their poor toughness. Particularly, when advanced composite materials are used as laminates, resistance to impact is governed in many cases by interlaminar fracture strength, so that use of thermosetting resins of poor toughness as the matrix results also in a reduction in impact resistance of the advanced composite materials. Because of this, use of the advanced composite materials, particularly, as structural materials has been fairly limited.
For improving the defect of this thermosetting matrix resin, for example a method of adding a rubber component is known. However, a large amount of the rubber component needs to be added in order to obtain a sufficient toughness-improving effect, which results in a large reduction in heat resistance, solvent resistance, etc.
Further, a method of adding thermoplastic resins, particularly the so-called engineering plastics excellent in heat resistance and solvent resistance is proposed (Japanese Patent Application Kokai No. 61-212543, No. 61-228016, No. 58-134111, etc.). These references report that reduction in heat resistance, solvent resistance, etc. is inhibited as compared with the above case wherein the rubber component is added, but a large amount of the thermoplastic resin needs to be added in order to obtain a sufficient toughness. Even in this method, however, there yet remains serious problems that the impregnation properties of the resin for producing a prepreg and the tack level of the prepreg lower with an increase in viscosity of the whole system.
Further, these trials to improve the toughness of these thermosetting matrix resins themselves are not a very effective method from the standpoint of improving the interlaminar peeling strength of the laminate. Also, a method of distributing the fine particles of thermoplastic resins between the layers is proposed (Japanese Patent Application Kokai No. 1-110537) with the object of effectively improving the interlaminar peeling strength of the laminate. However, a large reduction in tack level of the prepreg tends not to be avoided, and further problems of complicated manufacturing process and quality control newly occur. Although trials to localize chopped fibers, milled fibers, or the like between the layers also are proposed for the same object, the same problems cannot be avoided, and also the effect of the trials is usually not sufficient.
Further, a method of inserting one kind of impact-absorbing layer called an interleaf between the layers also is proposed (for example, U.S. Pat. No. 3,472,730, Japanese patent Application Kokai No. 51-58484, No. 60-63229, No. 60-231738, etc.). In any of these references, however, the thickness between the layers becomes large to lower the fiber ratio of the prepreg and the heat resistance and handling property become poor, so that this method has not yet come to find wide and practical application.